I would try to investigate the reason behind the decline of the subordinate's performance. Once the reason is discovered, I would decide whether the reason is professional or personal. If it is a professional reason, I would then try to find out whether I can solve the problem that the person has been facing.

If it is a personal problem, I would keep an eye on the time that the person has lost due to the personal problem. If the time taken cannot be handled by the company, I would speak to the person about the same, and give him an idea that he or she should try to solve the personal problem on his or her own, so that productivity may be increased for the company.